The Kimberley Diamond Company Ellendale diamond collection

Peter Downes's blog | Created 5 years ago

In November 2011, the Kimberley Diamond Company and private benefactors, donated to the Western Australian Museum, a representative collection of yellow and white diamonds, to a total weight of 38.06 carats, from their operations at Ellendale in the West Kimberley region of Western Australia.

Diamonds were discovered at Ellendale in November 1976. The diamonds occurred in what was called the Ellendale 4 olivine lamproite pipe. This pipe is the remains of an old volcano that erupted an unusual olivine-rich rock, called lamproite, that had originated over 150 km down below the Earth’s surface, within the stability field of diamond. More than 100 lamproites have since been discovered in the West Kimberley, although few of them contain diamonds. Overshadowed by the discovery in 1979, of the extremely diamond-rich Argyle lamproite pipe in the East Kimberley, the Ellendale diamond field lay dormant until 2003, when the Kimberley Diamond Company started mining the Ellendale 9 olivine lamproite, and later, in 2005, the Ellendale 4 olivine lamproite. Since that time the Ellendale mine has become famous for its production of high quality fancy yellow diamonds that are now sold through Tiffany and Co., New York. Presently the Ellendale mine contributes approximately 50% of the world production of fancy yellow diamonds. The unusual geological setting of the Ellendale mine, hosted by olivine lamproites emplaced along the margin of the Kimberley Craton (similar to the setting of the Argyle diamond mine), and its production of a unique range of white and yellow diamonds, give it an important place in the natural heritage of Western Australia.

The Ellendale diamond collection includes eight rough diamonds that range in weight from 0.97–3.69 carats, and in colour, from white to off-white, yellow and fancy yellow. Two specimens are diamonds in lamproite matrix, and the remainder of the collection consists of mixed parcels of small white, yellow, grey and brown stones. The diamonds are predominantly resorbed, with lustrous, smooth surfaces, and include dodecahedra, irregularly-shaped stones and minor macles. Yellow colours in the Ellendale diamonds are produced by their nitrogen content that is generally in the range ~100–1000 parts per million. Recent estimates suggest that the Ellendale diamonds formed about 1426 million years ago and were stored in a stable region of the Earth’s mantle, at temperatures of ~1100–1200˚C, until their eruption approximately 20 million years ago.

For yellow diamonds, which form a high proportion of stones from the Ellendale lamproites, the presence of nitrogen atoms in the diamond structure causes absorption in the blue region of the visible light spectrum, meaning that transmission of the remainder of the spectrum produces a yellow colour. The Ellendale 9 lamproite pipe is characterised by the highest proportion of fancy yellow diamonds occurring in any known kimberlite or lamproite worldwide. These high value diamonds contribute greater than 75% of the total value extracted from this lamproite, yet comprise approximately only 10% of the total carat production, occurring at a grade of just 0.6 carats per hundred tonnes of lamproite. Overall diamond grades at the mine are aproximately 5–7 carats per hundred tonnes.

The donation of the the Kimberley Diamond Company Ellendale diamond collection to the Western Australian Museum has allowed the preservation of an important part of the rich mineral heritage of Western Australia. In combination with the museum’s Argyle Diamonds pink diamond collection, the Ellendale collection provides a valuable display resource of diamonds from the Kimberley region that will benefit the people of Western Australia, and visitors to the State, in perpetuity.